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Blast rocks Pemex tower in Mexico City

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Of those injured, at least 28 remained hospitalized Monday night, according to the health ministry.

Thousands of people work at the company's headquarters, which includes a 54-story building that is nearly 700 feet tall. The annex where the explosion happened is adjacent to the tower.

Employees at the annex said that the basement of the building, where the explosion originated, housed a water treatment facility, storage areas and archives. The first floor, which also was heavily damaged, is where the company's human resources team worked.

A makeshift memorial for the victims has sprung up outside the complex, where handwritten notes, flowers and candles line a fence.